Conventional photomask blanks commonly consist of a substrate, e.g., fused silica plate, on which is an opaque chrome film. Photomasks are produced from these blanks by providing a desired pattern of open areas in the film. In use, light is optically projected through the open areas of the photomask onto the surface of a light sensitive substrate, such as a photopolymer-coated semiconductor wafer. Currently, photomasks are illuminated with visible or ultraviolet light. A fundamental limitation of optical imaging is that line widths of the order of the wavelength of the illuminating light are diffraction limited. In other words, light having a wavelength of the same order of magnitude as the desired optical image will diffract, and the projected image will be wider than the desired image.
To allow for patterns narrower than the wavelength of visible or ultraviolet light to be produced, the art has considered changing to a shorter wavelength source, such as X-ray. U.S. Pat. No. 4,890,309 discloses a type of attenuated (as opposed to opaque) photomask, particularly useful in X-ray lithography, wherein the material and thickness of the attenuating film are selected to both pass (i.e., transmit) a fraction of the incident electromagnetic radiation and to phase shift the radiation relative to radiation passing through the open features of the mask. The materials of choice are homogeneous films of tungsten, gold, silver, alloys of these materials or alternating layers of high and low atomic number materials (e.g., tungsten and carbon).
In an effort to avoid the cost and complexity of X-ray lithography, a variety of phase-shifting photomasks have been developed for ultraviolet and visible light ranges (see, for example, B. J. Lin, Solid State Technology, pp. 43-47, January, 1992). Among these are the rim phase-shifting photomask, which requires either substrate etching or the use of an additional phase-shifting layer. The rim phase-shifting photomask is inherently applicable to arbitrary mask patterns, but requires a large positive mask bias to reduce exposure times to a practical level and due to strong proximity effects it is difficult to delineate all feature sizes and shapes for an arbitrary mask pattern using one common exposure. An improvement over the rim phase-shifting photomask is known in the art as the attenuated phase-shifting photomask (APSPM). APSPMs employ an absorptive, partially transmitting, phase shifter in place of the conventional opaque chromium part of the patterned film. The transmission of the absorptive phase shifter is adjusted to less than about 0.20 to prevent creation of ghost lines. However, not all phase shifters can phase shift and absorb by the desired amount. Consequently, a multilayered structure consisting of materials of differing absorptive and phase shifting properties may be required in some cases. A commercially available APSPM utilizes a graded chromium oxycarbonitride composition film which varies from a Cr-N compound at the substrate-film interface to a Cr-O compound at the film-air interface, which also serves as an anti-reflective coating. While this APSPM provides a degree of phase shifting, a further procedure, such as reactive ion etching of the substrate, in this case, fused silica, is necessary to achieve the desired 180.degree. phase shift.
Japanese laid-open patent application No. Heisei 5-127,361 describes an attenuating photomask blank with a partially transmitting film that provides both the desired transmission and phase shift, i.e., the phase shift is embedded in the absorbing layer. This is achieved by the use of a depthwise optically homogeneous film material with appropriate index of refraction and extinction coefficient. A disadvantage of this depthwise homogeneous film is that, because the index of refraction and extinction coefficient are coupled, the transmission, phase shift, and reflectivity cannot be separately selected. In general, lithographic imaging performance is affected by transmission, phase shift, and reflectivity, so independent control of these three parameters is desirable. Another disadvantage of this homogeneous film is that when chromium compounds are used as specified in the claims, the electrical resistivity is high, rendering the film unwritable in an electron beam patterning system unless a separate charge dissipation layer is used. A further disadvantage of this homogeneous film is that when chromium compounds are used, the spectral optical transmission is very high in the visible portion of the spectrum, causing difficulty in inspecting the finished photomask (customarily done at 488 nm) and aligning the finished photomask in the projection aligner (customarily done at 633 nm).
The use of a depthwise optically inhomogeneous film to simultaneously achieve the desired transmission, phase shift, and reflectivity in an attenuated phase shifting photomask blank is described in U.S. patent application Ser. No. 07/976,782, now abandoned, filed Nov. 16, 1992. The inhomogeneous attenuating film consists essentially of a combination of a metallic component and a dielectric component, wherein one surface of the film has a higher content of metallic component than the other surface, and the profile of change in the extinction coefficient is gradual through the film thickness; and wherein said profile of change and the film thickness are selected to provide a phase shift of about 180.degree. (or an odd multiple thereof) at the selected wavelength. This inhomogeneous film allows flexible design and construction of photomask blanks with independent control of transmission, phase shift, and reflectivity. However, it specifies a depthwise composition gradient between a metallic component and a dielectric component.